166 



TENSION OF GASES. 



just supports tliis pressure and must therefore be f atmospheres. 

 The volume of the confined air is only f what it was under a pres- 

 sure of one atmosphere. If more mercury be poured into the long 

 arm" until the vertical distance between the two mercurial surfaces 

 is one-half the height of the barometric column, the pressure and 

 tension will be -| atmospheres ; the volume of the confined air will 

 be | what it was under a pressure of one atmosphere. When mer- 

 cury has been poured into the long arm until the vertical distance 

 CA is equal to the height of the barometric column, the pressure 

 and tension will be two atmospheres, and the volume of the confined 

 air will be one-half what it was under a pressure of one atmos- 

 phere. The law has been thus " verified " up to 27 atmospheres, 

 notwithstanding which it is not considered rigorously exact. The 

 deviation from exactness, however, can be detected only by meas- 

 urement of great precision. 



286. The Rule Works both Ways. The law 

 holds good for pressures of less than one atmosphere, for 

 rarefied air as well as for compressed 

 air. To show that this is true, nearly 

 fill a barometer tube with mercury and 

 invert it over a mercury bath held in a 

 glass tank as shown in the figure. 

 Lower the tube into the tank until the 

 mercury levels within the tube and 

 without it are the same. The air in the 

 tube is confined under a pressure of one 

 atmosphere. Note the volume of air in 

 the barometer tube. Raise the tube 

 until this volume is doubled. The 

 vertical distance between the two mer- 

 curial surfaces will be found to be half 

 the height of the barometric column. 

 The confined portion of air, which is 

 now subjected to the pressure of half an 

 FIG. ioi. atmosphere, occupies twice the space it 



